Altimeters are a class of instruments that indicate an estimated altitude. Altimeters are commonly used in aircraft, though they can also be used in other circumstances, such as skydiving and surveying. There are different kinds of altitude, such as true altitude (the height above mean sea level) and absolute altitude (height above ground level in the local area). Unless otherwise indicated, altitude, as used herein, generally refers to true altitude (the height above mean sea level).
Different kinds of altimeters are known in the art. Each has advantages and disadvantages. For example, a radar-based altimeter can estimate absolute altitude, but may require additional information to calculate an estimated true altitude and can have issues with uneven terrain or foliage. A global positioning system (GPS) can provide true altitude, but GPS may not always be available, such as due to obstruction of the satellite signals. Barometric pressure may also be used to estimate true altitude (as barometric pressure generally decreases as altitude increases), but can be affected by local weather conditions, such as high pressure or low pressure zones and temperature. Thus, a need exists for an aircraft altimeter with increased accuracy over a large altitude range.